Delay line



J. E. MAY, JR

May 2, 1961 DELAY LINE Filed. July 6, 1959 DELAY SECTION FIG.

FIG. 2

DELAY SECT/ON 2 0 o m nod OQm nmd omd mod 09w RELATIVE FREQUENCY 017v FREQUE NCY MEGACYCLES INVENTOR J. E. MAY JR. 8)- W i ATTORNEY United States Patent Othce Patented May 2, 1961 DELAY LINE John E. May, Jr., Whippany, NJ., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed July 6, 1959, Ser. No. 825,326

Claims. (Cl. 333-30) A further object is to avoid the propagation of unwanted modes which give rise to spurious signals in the electrical output.

In wave transmission systems, it is sometimes desired to introduce a delay characteristic which is linear over a considerable frequency range and has a positive, negative, or zero slope. Ultrasonic delay lines are Well suited for this purpose. In order to improve the performance of the delay line, it is desirable to avoid spurious signals.

The delay line in accordance'with the present invention meets these requirements by employing two or more sections of solid, ultrasonic transmission line connected in tandem. Each of the sections has a transverse dimension comparable to the acoustic wavelength of the transmitted signal. The delay-frequency characteristic of one of the sections has a positive curvature, and that of a second section a negative curvature, over the frequency range of interest. The curvatures are so chosen that the over-all characteristic is substantially linear and has the desired slope.

The sections may employ the same or dilierent modes of propagation. Modes are chosen which provide the desired curvatures. It is also helpful to choose a mode which can be excited preferentially. Some modes have a characteristic with one or more points of inflection, and hence exhibit both positive and negative curvatures. These include all longitudinal and flexural modes. Therefore, one of these modes may be used in both sections.

However, the characteristics of the shear and torsional modes above the lowest have only a positive curvature. Consequently, if one of these modes is used in one of the sections, the second section must employ some other mode exhibiting a negative curvature in orderto obtai a linear over-all characteristic.

Spurious signals to be avoided may be those associated with undesired modes of propagation, or non-propagated modes associated with resonance effects at one or more frequencies. The desired mode may be favored overthe undesired by choosing the size of the electromechanical transducer so that its displacement configuration most closely approximates that of the desired mode. Regions where the characteristic of the desired mode coincides with that of an undesired mode are avoided.

A typical example of a composite delay line in accordance with the present invention is presented. The structure comprises two sections connected in tandem. Each section includes a length of wire with associated electromechanical transducers at the ends. The first longitudinal mode is used in each section. The delay-frequency characteristic of one of thesections has a positive curvature and that of the other a negative curvature over the frequency range of interest. These are so chosen'that the modes.

p and diameter of the delay line section 11; The delay- 'frequency curves of Fig; 3 are useful for this purpose.

over-all characteristic is substantially linear over the range and has a positive slope. Since the slope is not zero, the line is dispersive. In order to favor the first longitudinal mode over the other unwanted modes, the transducer comprises a cylinder of piezoelectric material having a smaller diameter than the wire line. Also, for the same reason, the adjoining ends of the crystal and the wire are made as nearly concentric as possible. To minimize reflection, the line preferably tapers at the end to the diameter of the transducer. Points where the characteristic of the first longitudinal mode is crossed by that of the second longitudinal mode, or the first, second, or third flexural mode, are avoided. Points at which nonpropagated, resonance effects appear are also avoided.

The nature of the invention and its various objects,

features, and advantageswill appear more fully in the following detailed description of a typical embodiment tive frequency for the first two longitudinal and the first 7 three flexural modes in a solid'cylindrical line; and

Fig. 4 shows plots of the departure from linearity ,versus frequency for a two-section delay line in accordance with the invention, and for each section.

Fig. 1 shows a composite delay line comprising two sections 1 and 2 connected-in tandem between a pair of input terminals 3-4 and a pair of output terminalsJ-S, with intermediate terminals 5'-6. A source 9 of altermating-current signals to be delayed is connected to the input terminals, and a suitable load 10 to the output terminals.

In the present example, it is assumed that each delay section employs a length of wire-type delay line operating in the first longitudinal mode. Fig. 2 shows, to an enlarged scale, the preferred structure of section 1. Section 2 may be of similar construction, but with different dimensioning, as explained below. The section comprises a solid, cylindrical rod or wire 11 with two substantially identical electromechanical transducers 12 and 13 con- The wire 11 has a uniform diameter D except, at the ends,

which taper to a smaller diameter d. The transducer 12. comprises a cylinder 14 of piezoelectric material, such as barium titanate ceramic, having a length S approximately equal to a half wavelength in the material at the midband frequency. Its ends are covered by the electrodes 15 and 16, which are connected, respectively, to the terminals 3 and 4. The inductor 33 is connected in series with the cylinder 14 to widen the transmission band of the transducer by providing a better electrical impedancematch. Its inductance is chosen to annul the effect of the reactive component of the input impedance of the transducer, at approximately the midband frequency.

When the signal is applied to the terminals 3- and 4, the transducer 12 excites the first longitudinal mode in the line 11. The diameter d of the driving cylinder 14 is made smaller than the diameter D of the line 11, and

the cylinder is carefully centered on the end of the line,

to reduce the generation of undesired flexural and other To reduce reflection, the ends ofthe line 11 are, preferably tapered at an angle of about 30 degrees.

Itwill now be explained how to determine the length .lus to .the density ,of the wire.

The ordinates represent the specific delay ratio, which is the ratio of the bar velocity V to the group velocity U. V is the square root of the ratio of Youngs modu- The abscissas are the frequency-multiplied ;by theratio of the ,diameterD of the wire to V The ,curves 17 ,and 22 are for the first and second longitudinal modes, respectively, and the curves 18, 28, and 29:for thefirst, second, and third flexural modes, respectively. It is seen that the curve 17 has a maximum at the point 19, with a positive slope below this frequency and a negative slope above. Also, the curve has two points of inflection and 21 between which the curvature is negative. Outside of this region, the curvature is positive. The curves shown are for aluminum with a Poissons ratio of 0.33.

The curves 18, 28, 29, and 22 crossthe curve 17 at the points 23, 30, 31, and 24, respectively. At the point 32 designated by X in Fig. 3, there is distortion in the amplitude and phase characteristic of the delay line, apparently caused by coupling with an interfering mode. All of these points arecritical and are to be avoided as operating regions for the delay sections. However, the shapes of the curves and the relative positions of the critical points vary with Poissons ratio. For example, for a nickel-iron alloy with a Poissons ratio of 0.23, the crossing point 23 occurs at approximately the same position with respect to the inflection point 20, but the critical point 32 occurs at a lower value of relative frequency.

In the present example, each of the delay sections 1 and 2 operates on a portion of the curve 17. In order to get a linear over-all delay-frequency characteristic, one of the sections operates somewhere between the lower inflection point 20 and the upper inflection point 21, Where the curvature is negative. The other section operates outside of this region, where the curvature is positive. To get a maximum positive slope, both of the sections will operate on the portion of the curve below the point 19, where the slope is positive. For a maximum negative slope, the portion above the point 19, with negative slope, is utilized. However, if maximum slope is not essential, only the section having the greater delay change need have the desired slope, either positive or negative. The other section may be of either slope. To get zero slope for the over-all characteristic both the slopes and the curvatures of the portions of the curve 17 used in the two sections 1 and 2 must differ in sign. Therefore, a portion below the point 20 may be combined with a portion between the points 19 and 21, or a portion above the point 21 may be combined with a portion between the points 19 and 20. In all cases, the crossing points 23, 30, 31, and 24, and also the critical point 32, are avoided in selecting the portion of the curve 17 to use.

In the present example, a substantially linear characteristic between 2.84 and 3.14 megacycles per second with approximately maximum positive slope is obtained by using two lengths of aluminum wire with a Poissons ratio of 0.33. The section 1 operates on the curve 17 in a region below the point 23 where both slope and curvature are positive. The section 2 uses the region of positive slope but negative curvature between the points and 31. The section 1 has a diameter of 0.027 inch and a length of 1868.4 inches. Its delay increases from 11,375 to 12,408 microseconds over the band, and is 11,857 at the midband frequency of 2.99. The section 2 has a diameter of 0.040 inch, a length of 157.1 inches, and delays of 1,766 and 1,996 microseconds, respectively, at the lower and upper edges of the band.

The curves in Fig. 4 show the performance of the composite delay line. The curve 25 gives the departure ofthe section 1 from linearity, and the curve 26 gives the departure of the section 2. Each has a maximum departure of about 44 microseconds, near midband, but

the departures are of opposite sign. The deviation of the combination, given by the broken-line curve 27, has a maximum of less than four microseconds over the band, and is zero at the edges and at two intermediate points. The over-all delay increases from 13,141 to 14,404 microseconds over the band. There is thus achieved a dispersive delay characteristic that is substantially linear over a comparatively wide frequency range and has a comparatively large positive slope.

As already mentioned, the two sections do not need to use the same mode of propagation. For example, a linear over-all delay characteristic with negative slope may be obtained by using a portion of the curve 18, for the first fiexural mode, to the left of the point 23 in one section and a portion of the curve 17, for the first longitudinal mode, between the points 19 and 21 in the other section. Or, alternatively, a portion of the curve 28, for the second flexural mode, between the crossing points 34 and 35 may be used in the second section.

It is to be understood that the above-described arrangement is only illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A composite delay line with a substantially linear delay versus frequency characteristic over a comparatively Wide frequency range comprising two sections connected in tandem, each of the sections including a solid, ultrasonic transmission line and associated electromechanical transducers at the ends thereof, one of the transmission lines having a characteristic of specific delay ratio versus relative frequency for its operating mode which has a positive curvature over the band and the other transmission line having a characteristic of specific delay ratio versus relative frequency for its operating mode which has a negative curvature over the band.

2. A delay line comprising two solid, ultrasonic transmission lincs connected in tandem, one of the transmission lines having a characteristic of specific delay ratio versus relative frequency for a selected mode of propagation which has a positive curvature over a band of frequencies,the other transmission line having a characteristic of specific delay ratio versus relative frequency for a selected mode of propagation which has a negative curvature over the band, the curvatures being chosen to provide a linear overall delay-frequency characteristic, and the utilized portions of the characteristics being free from points of interference due to unwanted modes.

3. A composite delay line with a delay versus frequency characteristic which is substantially linear over a frequency range and has a positive slope comprising two lengths of solid transmission line connected in tandem, the first length providing the major delay change and having a characteristic of specific delay ratio versus relative frequency with a positive slope over the range and the second length having a characteristic of specific delay ratio versus relative frequency with a curvature which differs in sign from that of the first length over the range.

4. A delay line in accordance with claim 3 in which the second length has a positive slope over the range.

5. A delay line in accordance with claim 3 in which the second length has a negative slope over the range.

6. A composite delay line with a delay versus frequency characteristic which is substantially linear over a frequency range and has a negative slope comprising two lengths of solid transmission line connected in tandem, the first length providing the major delay change and having a characteristic of specific delay ratio versus relativefrequency with a negative slope over the range and the other length having a characteristic of specific delay ratio versus relative frequency with a curvature which diflers in sign from that of the first length over the range.

7. A delay line in accordance with claim 6 in which the second length has a negative slope over the range.

D 8. A delay line in accordance with claim 6 in which the second length has a positive slope over the range.

9. In combination, two sections of solid, ultrasonic transmission line connected in tandem, the sections having characteristics of specific delay ratio versus relative frequency which difier from each other both in the sign of the slope and the sign of the curvature over a frequency range, and the utilized portions of the characteristic being so selected that the over-all delay versus frequency characteristic of the combination is substantially flat over the range.

10. A dispersive delay line comprising two lengths of ultrasonic transmission line connected in tandem, each of the lengths being adapted to operate in the first longitudinal mode and having a characteristic of specific delay ratio versus relative frequency with a point of inflection,

References Cited in the file of this patent UNITED STATES PATENTS 2,181,499 Wheeler Nov. 28, 1939 2,697,936 Farrow Dec. 28, 1954 2,806,155

Rotkin Sept. 10, 1957 

